Abstract: An electron beam source includes a cathode having an electron emission surface including an active area for emission of electrons and a cathode shield assembly including a conductive shield disposed in proximity to the electron emission surface of the cathode. The shield has an opening aligned with the active area. The electron beam source further includes a device for stimulating emission of electrons from the active area of the cathode, electron optics for forming the electrons into an electron beam and a vacuum enclosure for maintaining the cathode at high vacuum. The cathode may be a negative electron affinity photocathode formed on a light-transmissive substrate. The shield protects non-emitting areas of the emission surface from contamination and inhibits cathode materials from contaminating components of the electron beam source. The cathode may be moved relative to the opening in the shield so as to align an new active area with the opening.